36 ELECTRICAL MEASUREMENTS 



Consequently, in very strong uniform fields, the attraction 

 between the magnet and the induced poles on the coil, when it is 

 deflected from its initial position, may be strong enough to mate- 

 rially reduce the sensitiveness. In some cases the control thus 

 exercised may be greater than that due to the suspension. The 

 magnetic action of the coil also gives rise to indefiniteness of the 

 zero reading which will be displaced in the direction of the next 

 previous deflection. These difficulties are reduced to negligible 

 amounts if a radial field is used as indicated in Fig. 16. 



Obviously, induced poles, if they exist, do not affect the restor- 

 ing moment when the coil changes its position. This con- 

 struction also gives a long and uniform scale. It was introduced 

 in the Weston direct-current ammeters and voltmeters in 1888. 



When the coil moves in a radial field, the turning moment 

 acting upon it is given by 



M = IHlb. 



Suspensions. The materials commonly used for the sus- 

 pension wires in commercial instruments are phosphor-bronze 

 and steel. Silver gives a low resistance wire but is not as stable 

 as phosphor-bronze. 



The sensitivity will be increased by diminishing the torsion 

 constant, T, which is proportional to the fourth power of the 

 diameter of the suspension wire. Of course, with a given coil, 

 the stress per unit area on the suspension is inversely as the 

 square of the diameter. 



To increase the sensitivity without increasing the unit stress 

 on the suspension, Ayrton and Perry suggested that a flat strip 

 be substituted for the round wire. A strip having a breadth of 

 about ten times its thickness has approximately one-fifth of the 

 torsional rigidity of a round wire of the same length and sectional 

 area. Such suspensions are very commonly used in commercial 

 moving coil galvanometers. Northrup has suggested the employ- 

 ment of a cable of very fine wires as a means of supporting heavy 

 coils. For a cable capable of supporting a given weight the 

 torsional rigidity decreases in proportion as the number of 

 strands is increased. This form of suspension is frequently used 

 in portable galvanometers. 



All connections about the suspended system should be soldered, 

 M otherwise extraneous resistance^ may be introduced by loose 



